A power amplifier is an indispensable part of a wireless base station, and the efficiency of the power amplifier is a decisive factor that affects the efficiency of the base station. Currently, to improve the utilization of a frequency spectrum, modulated signals of many different standards are used in wireless communication, and these modulated signals have different peak-to-average ratios. Signals with a high peak-to-average ratio have high requirements on the power amplifier of the base station, and therefore, to perform amplification processing on these signals with a high peak-to-average ratio without distortion, the power amplifier of the base station may use a power back-off method, which, however, leads to low efficiency. High-efficiency power amplifiers mainly include an envelope tracking amplifier and a Doherty (Doherty) power amplifier.
For the envelope tracking power amplifier, operating voltage of the amplifier is determined according to envelope amplitude of an output radio-frequency signal: When radio-frequency output power is low, a low voltage is used to supply power, and the voltage increases as the radio-frequency output power increases, so that the amplifier can operate in a near-saturation area corresponding to the corresponding power supply voltage for different power inputs, thereby reducing power loss, and improving power back-off efficiency. An advantage of the envelope tracking power amplifier lies in that, an envelope modulator is irrelevant to the radio-frequency operating frequency, and can achieve a wide radio-frequency operating bandwidth. However, the overall operating efficiency of the envelope tracking power amplifier is equal to a product of the operating efficiency of the envelope modulator and the operating efficiency of a radio-frequency power amplifier. Because the operating efficiency of the envelope modulator cannot be 100%, there is some loss in the overall operating efficiency of the envelope tracking power amplifier. Especially in a case in which a modulated signal has a wide bandwidth, the envelope modulator is not easily implemented, the efficiency decreases as envelope bandwidth increases, and the loss in the overall operating efficiency of the envelope tracking power amplifier is higher, and therefore it is very difficult to implement efficient power amplification of a modulated signal having a wide bandwidth.
The Doherty (Doherty) power amplifier improves the power back-off efficiency by using an active-load pulling method. That is, load impedance of a primary power amplifier of the Doherty power amplifier is pulled by an auxiliary power amplifier, so that the primary power amplifier can reach a saturated state in advance in a case of low input power and output power, and keep operating in the saturated state as the input power continues to increase, thereby improving the power back-off efficiency. The Doherty power amplifier does not have an envelope modulator, and therefore has a higher modulated signal bandwidth than that of the ET power amplifier. However, in the Doherty power amplifier, the combined output circuit design is relevant to the operating frequency, and after the operating frequency deviates from a set center frequency, the operating state deviates from the optimal active-load pulling state, which reduces the efficiency. Therefore, a wide radio-frequency operating bandwidth is not easily achieved.